BCP47 defines two level of conformance: well-formed and valid.

The latter of which is expensive, as it involves checking against possibly the entire subtag registry for a given language tag. While there are no pressing reasons to implement such an API, filing this issue as a place-holder for any possible future considerations.

Such a change would have negative impact on build-time, which is when the subtag registry is processed. This would be the primary road-block at first glance.